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Crystallization of La2-xNaxCuO4 superconductor by low-temperature electrochemical deposition
PHYSICA Physica C 231 (1994) 325-329
ELSEVIER
Crystallization of Laz_xNaxCuO4 superconductor by low-temperature electrochemical deposition H.Y. Tang, C.S. Lee, M.K. Wu Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan
Received 17 June 1994; revised manuscript received 12 August 1994
Abstract
Crystalline La2_xNaxCuO4 was obtained by a low-temperature anodic electrocrystallization method in a sodium hydroxide molten flux. A 2 h experiment can produce plate-like crystals with 0.1 mm × 0.1 mm facets in a-b-direction and 32 K superconductivity as-grown.
1. Introduction
The electrodeposition of cuprate-based superconductors has considerable potential in future applications. In comparison with other coating techniques, electrodeposition offers many advantages such as low cost, large scale production, and rapid coating on nonplanar surfaces. Previous studies on the preparation of superconducting thick films concentrated on reducing materials at the electrode surface with a rather negative reduction potential, which limits most of the solvents to nonaqueous media [ 1,2]. These e!ectrodeposited products do not have the perovskite-related structure, and need sintering at high temperature to convert them to the perovskite-related structure, followed by annealing to properly adjust the oxygen content. The low-temperature electrochemical approach is an alternative method to solve the high-temperature problems. The low-temperature molten salt anodic electrocrystallization method reported by Norton [ 3 ] has a number of advantages in the preparation of superconducting Ba~_xKxBiO3 crystals at 260°C and overcomes the difficulties of high-temperature treatment.
In the KENiF4 system, the semiconductor La2CuO4 can be converted to a superconductor LaECuO4+x by a high-pressure oxygen treatment [4] or by electrochemical oxidation in aqueous solution [5,6]. Chemical substitutions of alkaline earth [ 7,8] or alkaline metals [ 9 ] at the lanthanum site were found to generate superconductivity with Tc near 40 K. However, most of the processes involve high-temperature sintering and none of them can readily form superconductive coatings at low temperature. A lowtemperature, nonelectrochemical method of precipitating superconducting LaE_x(Na, K)xCuO4 was published in 1988 [ 10 ]. Our research has focused on the electrochemical characteristics of this system. In this paper, we report a low-temperature electrochemical method which can synthesize crystalline La2_xNa:,CuO4 thick layers with 32 K as-grown superconductivity in a relatively short reaction period.
2. Experimental
La2_xNaxCuO4 crystals were grown by electrolytic oxidation of a molten sodium hydroxide solution
0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All fights reserved SSD10921-4534 (94)00521-4
326
H.Y. Tang et al. / Physica C231 (1994) 325-329
system. A schematic of the crystal growth system is shown in Fig. 1. It consists of a vacuum chamber to prevent air oxidation and has the flexibility to perform a variety of experiments. Molten salts generally can be considered as ultra-concentrated electrolyte solutions. The extremely corrosive nature of molten hydroxides limit material selection for containers. In an inert gas atmosphere, platinum crucibles exhibited long-time stability for sodium hydroxide melt but corroded rapidly in air. In this work, low-temperature electrocrystallization experiments have been carried out in commercial platinum containers. Depositions were performed under galvanostatic control, with 1 mm diameter Ag and Pt wire as anode and cathode, respectively. Potentiostatic studies were performed utilizing a three-electrode, one-compartment cell geometry. A EG&G 273A high-current potentiostat was employed for these studies. The working electrode can be a platinum or silver wire. A 1 mm diameter silver wire electrode was most commonly used. The counter electrode used was a 1 mm diameter platinum wire. Platinum wire was used as a pseudo-reference electrode, and was placed in close proximity to the working electrode. In a typical deposition, 30 g of NaOH white pellets (Johnson Matthey), 0.4 g of CuO and 3.26 g of La203 are added in a 30 cm 3 platinum crucible and lowered into the furnace. A transparent melt was usually obtained around 320°C using commercial sodium hy-
Electrodes Support•~,~
II/•f°r
ElectricalContact Electrodes
droxide. Maintenance of an inert atmosphere is necessary in order to avoid oxidation of the chemical system. This oxidation can be quite rapid at elevated temperatures. The vacuum chamber was evacuated to 5 × 10- 2 Torr and then backfilled with argon gas for ensuring minimum spontaneous oxidation. After reaching the operating temperature, constant current or constant potential was applied to initiate the electrochemical reaction. Morphological examination and photomicrographs of the crystals were taken using a Jeol JSM840A scanning electron microscope (SEM). Nondestructive micro-elemental analysis was performed utilizing a Link exl data acquisition system for collection of the energy dispersive X-ray (EDX) spectra and for analysis utilizing the system's semiquantitative analysis program. X-ray diffraction patterns of these powders were taken utilizing a MAC MXP3 Xray diffraction system. Transition temperature was determined using a Quantum Design superconducting quantum interference device (SQUID) magnetometer.
3. Results and discussion
The complex ions in molten hydroxide flux display a strong tendency to react with oxygen and moisture. The interaction of water with these melts in the light of the Bronsted theory was described by Goret [ 11,12 ]. Hydroxide ions in the melt dissociate into water and oxide ions 2 O H - ~ H 2 0 + O 2- .
•IL
111 Ir
Pt ~cible
,l.. '~Melt
Fig. 1. Schematic representation of the electrochemicalcrystal growthsystem.
The acid-base properties of hydroxide melts are defined by pH=pH20= -log[H20] . The activity of 0 2 - in hydroxide flux is highly related with the PH20 value at constant temperature. Unfortunately, the disadvantages of the molten hydroxides include not only extremely corrosive properties but also low accuracy of pH20 control. These limit the crucible materials and reaction conditions in electrodeposition experiments. Two simplified conditions, acidic (highly hydrated) and basic (dehydrated) are adopted in this study. High accuracy of pH20 control is not attempted.
H.Y. Tang et aL / Physica C231 (1994) 325-329
Under acidic conditions, experiments were performed in a three-electrode quartz cell as described elsewhere [ 3 ]. A water-vapor-saturated argon gas flow was maintained over the cell to prevent oxidation and maintain a near constant pH20 at 320"C. The equilibrium between the molten flux and the partial pressure of water in the gas had been reported by Rahmel [ 13 ]. Under these acidic conditions, dark brown crystals were formed on the anode in 3 h of electrolysis when the current density was controlled at 1 mA/cm 2. An SEM micrograph of representative crystallites is shown in Fig. 2(a). The CuO structure of these crystallites was confirmed by X-ray diffraction analysis as shown in Fig. 3 (a). For the basic conditions experiment, water is removed by maintaining the molten flux at 450"C for 12 h in a quartz cell under a stream of dry argon. After complete dehydration, the platinum crucible with the charge of oxides materials is quickly transferred into the vacuum chamber. Electrolysis at 450"C of the basic melt with 1 mA/cm 2 current density gave a rapid deposition of plate-like crystals shown in Fig. 2(b) which proved to be La2_xNaxCuO4. A current density greater than 5 mA/cm 2 results in CuO crystallite deposition on the anode, indicating that the diffusion of La complex ions is important. After 2 h of electrolysis at 1 mA/cm 2 constant current, the average crystal size is approximately 100 ~tm as shown in Fig. 2(c). X-ray diffraction studies shown in Fig. 3(b) indicate that the crystallites contain a La2CuO4-related phase with the growth direction along the a-bplane. Averaging the EDX analysis on several crystals, yields a composition in atom percent of the crystals of 66.8% La and 33.2% Cu which is in good agreement with ICP-AES results. At the low concentration of alkali metals in this system, we do not consider the EDX results reliable for sodium determination. Same as in the ICP-AES analysis, the accuracy of elemental sodium may not be well represented by the precision of the measurement. However, an x value smaller than 0.08 is consistently obtained in all of the ICP-AES analysis. Fig. 4 displays the typical SQUID data of the asgrown crystals which are harvested by scratching crystals out from the electrode surface. The results show that the samples are superconducting with an onset temperature ~ 32 K. The estimated superconducting volume fraction is more than 60%, suggest-
327
(a)
(b)
(c) Fig. 2. Photomicrographsof (a) CuO (b) La=_=Na=CuO4+=prepared by 10 rain of electrolysis, (c) La2_xNaxCuO4+xprepared by 2 h of electrolysisdepositedon the electrodesurface. ing bulk superconductivity of the crystals. However, sample compositional inhomogeneity exists as exemplified by the rather broad transition and the paramagnetic-like contribution to the susceptibility in the field-cooled measurement. Cyclic voltammetry is not
H.Y. Tang et al. / Physica C231 (1994) 325-329
328
L a 2 _ x N a x f u O 4 in the deposit, indicating that diffu-
2500
sion of lanthanum complex ions acts as a fundamental step in the superconducting phase formation. Higher molecular weight hydroxide flux deposition does not appear practical in view of our studies of the electrochemical crystal growth of this material. A similar study performed using a KOH molten flux has not been found to have satisfactory results for La2_xKxCuO4 deposition. The intrinsic difference of electrochemical oxidation ability between KOH and NaOH may significantly impact the deposition mechanism.
2250 2000 1750 CuO
<-~ 1500 .~ 1250
2
lOOO
750 500 250 0 10
20
30
40
50
60
70
80
90
4. Conclusion
2 theta Fig. 3. Typical powder X-ray diffraction patterns of (a) CuO (b) La2_xNaxCuO4.x crystallites.
0890000 ~.
-2
o
70
o
oF C °
o
o o o o
-4
Crystallization of La2_ xNaxCuO4by a low-temperature, isothermal electrochemical method has been demonstrated. This technique offers the distinct advantage of superconductor synthesis through electrochemical control and has considerable potential for the synthesis of ternary or even multinary high-To materials. In this preliminary research, temperature, pH20, flux composition, and electrolytic conditions are correlated with product formation. Detailed studies to clarify electrochemical interaction among these factors are proceeding.
0
-6 0
ZFC
Acknowledgements
0
H=20G
-8
0 0 -I0
i
I0
i 20
i
30
40
T e m p e r a t u r e (K)
The authors thank S.M. Rao and M.L. Norton for helpful discussion. This work is supported by the ROC National Science Council Grant NSC83-0511M-007-004.
Fig. 4. SQUID measurement for the La2_xNaxCuO4+x crystals grown from low-temperature molten flux.
able to provide valuable information about the possible electrochemical reaction mechanism in both the acidic and basic experiments due to reference electrode instability and low decomposition potential in the basic environment. Potentiostatic deposition experiments were performed at 0.075 V in the basic melt with a La203 to CuO molar ratio range from 2 to 10. La2_xNa~CuO4 and CuO mixed-phase growth at the electrode surface was found in all cases after 2 h of electrolysis. Higher La/Cu molar ratios display more
References [1] R.N. Bhattacharya, R. Noufi, L.L. Roybal and R.K. Ahrenkiel, J. Electrochem. Soe. 138 ( 1991 ) 1643. [2] J.M. Rosamilia and B. Miller, J. Electrochem. Soc. 136 (1989) 1053. [3] M.L. Norton and H.Y. Tang, Chem. Mater. 3 ( 1991 ) 431. [4] P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phy. Rev. Lett. 58 (1987 ) 2482. [ 5 ] J.C. Bennett, M. Olfert, G.A. Seholz and F.W. Boswell, Phys. Rev. B 44 (1991) 2727.
H.Y. Tang et aL /Physica C 231 (1994) 325-329 [6]J.C. Grenier, A. Wattiaux, N. Lagueyte, J.C. Park, E. Marquestaut, J. Etoumeau and M. Pouchard, Physica C 173 (1991) 139. [7] J.G. Bednorz, M. Takashige and K.A. Muller, Europhys. Lett. 3 (1987) 379. [8] G. Demazeau, F. Tresse, Th. Plante, B. Chevalier, J. Etourneau, C. Michel, M. Hervieu, B. Raveau, P. Lejay, A. Sulpice and R. Tournier, Physica C 153 (1988) 524.
329
[9] M.A. Subramanian, J. Gopalakrishnan, C.C. Torardi, T.R. Askew, R.B. Flippen, A.W. Sleight, J.J. Lin and S.J. Pooh, Science 240 (1988) 495, [10] W.K. Ham, G.F. Holland and A.M. Stacy, J. Am. Chem. Soc. 110 (1988) 5214. [ 11 ] J. Goret, Bull. Soc. Chim. (1974) 1074. [ 12] J. Goret and B. Tremillon, Bull. Soc. Chim. (1966) 67. [ 13 ] A. Rahmel and H.J. Kruger, Z. Phys. Chem. 55 (1967) 25.
ELSEVIER
Crystallization of Laz_xNaxCuO4 superconductor by low-temperature electrochemical deposition H.Y. Tang, C.S. Lee, M.K. Wu Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan
Received 17 June 1994; revised manuscript received 12 August 1994
Abstract
Crystalline La2_xNaxCuO4 was obtained by a low-temperature anodic electrocrystallization method in a sodium hydroxide molten flux. A 2 h experiment can produce plate-like crystals with 0.1 mm × 0.1 mm facets in a-b-direction and 32 K superconductivity as-grown.
1. Introduction
The electrodeposition of cuprate-based superconductors has considerable potential in future applications. In comparison with other coating techniques, electrodeposition offers many advantages such as low cost, large scale production, and rapid coating on nonplanar surfaces. Previous studies on the preparation of superconducting thick films concentrated on reducing materials at the electrode surface with a rather negative reduction potential, which limits most of the solvents to nonaqueous media [ 1,2]. These e!ectrodeposited products do not have the perovskite-related structure, and need sintering at high temperature to convert them to the perovskite-related structure, followed by annealing to properly adjust the oxygen content. The low-temperature electrochemical approach is an alternative method to solve the high-temperature problems. The low-temperature molten salt anodic electrocrystallization method reported by Norton [ 3 ] has a number of advantages in the preparation of superconducting Ba~_xKxBiO3 crystals at 260°C and overcomes the difficulties of high-temperature treatment.
In the KENiF4 system, the semiconductor La2CuO4 can be converted to a superconductor LaECuO4+x by a high-pressure oxygen treatment [4] or by electrochemical oxidation in aqueous solution [5,6]. Chemical substitutions of alkaline earth [ 7,8] or alkaline metals [ 9 ] at the lanthanum site were found to generate superconductivity with Tc near 40 K. However, most of the processes involve high-temperature sintering and none of them can readily form superconductive coatings at low temperature. A lowtemperature, nonelectrochemical method of precipitating superconducting LaE_x(Na, K)xCuO4 was published in 1988 [ 10 ]. Our research has focused on the electrochemical characteristics of this system. In this paper, we report a low-temperature electrochemical method which can synthesize crystalline La2_xNa:,CuO4 thick layers with 32 K as-grown superconductivity in a relatively short reaction period.
2. Experimental
La2_xNaxCuO4 crystals were grown by electrolytic oxidation of a molten sodium hydroxide solution
0921-4534/94/$07.00 © 1994 ElsevierScienceB.V. All fights reserved SSD10921-4534 (94)00521-4
326
H.Y. Tang et al. / Physica C231 (1994) 325-329
system. A schematic of the crystal growth system is shown in Fig. 1. It consists of a vacuum chamber to prevent air oxidation and has the flexibility to perform a variety of experiments. Molten salts generally can be considered as ultra-concentrated electrolyte solutions. The extremely corrosive nature of molten hydroxides limit material selection for containers. In an inert gas atmosphere, platinum crucibles exhibited long-time stability for sodium hydroxide melt but corroded rapidly in air. In this work, low-temperature electrocrystallization experiments have been carried out in commercial platinum containers. Depositions were performed under galvanostatic control, with 1 mm diameter Ag and Pt wire as anode and cathode, respectively. Potentiostatic studies were performed utilizing a three-electrode, one-compartment cell geometry. A EG&G 273A high-current potentiostat was employed for these studies. The working electrode can be a platinum or silver wire. A 1 mm diameter silver wire electrode was most commonly used. The counter electrode used was a 1 mm diameter platinum wire. Platinum wire was used as a pseudo-reference electrode, and was placed in close proximity to the working electrode. In a typical deposition, 30 g of NaOH white pellets (Johnson Matthey), 0.4 g of CuO and 3.26 g of La203 are added in a 30 cm 3 platinum crucible and lowered into the furnace. A transparent melt was usually obtained around 320°C using commercial sodium hy-
Electrodes Support•~,~
II/•f°r
ElectricalContact Electrodes
droxide. Maintenance of an inert atmosphere is necessary in order to avoid oxidation of the chemical system. This oxidation can be quite rapid at elevated temperatures. The vacuum chamber was evacuated to 5 × 10- 2 Torr and then backfilled with argon gas for ensuring minimum spontaneous oxidation. After reaching the operating temperature, constant current or constant potential was applied to initiate the electrochemical reaction. Morphological examination and photomicrographs of the crystals were taken using a Jeol JSM840A scanning electron microscope (SEM). Nondestructive micro-elemental analysis was performed utilizing a Link exl data acquisition system for collection of the energy dispersive X-ray (EDX) spectra and for analysis utilizing the system's semiquantitative analysis program. X-ray diffraction patterns of these powders were taken utilizing a MAC MXP3 Xray diffraction system. Transition temperature was determined using a Quantum Design superconducting quantum interference device (SQUID) magnetometer.
3. Results and discussion
The complex ions in molten hydroxide flux display a strong tendency to react with oxygen and moisture. The interaction of water with these melts in the light of the Bronsted theory was described by Goret [ 11,12 ]. Hydroxide ions in the melt dissociate into water and oxide ions 2 O H - ~ H 2 0 + O 2- .
•IL
111 Ir
Pt ~cible
,l.. '~Melt
Fig. 1. Schematic representation of the electrochemicalcrystal growthsystem.
The acid-base properties of hydroxide melts are defined by pH=pH20= -log[H20] . The activity of 0 2 - in hydroxide flux is highly related with the PH20 value at constant temperature. Unfortunately, the disadvantages of the molten hydroxides include not only extremely corrosive properties but also low accuracy of pH20 control. These limit the crucible materials and reaction conditions in electrodeposition experiments. Two simplified conditions, acidic (highly hydrated) and basic (dehydrated) are adopted in this study. High accuracy of pH20 control is not attempted.
H.Y. Tang et aL / Physica C231 (1994) 325-329
Under acidic conditions, experiments were performed in a three-electrode quartz cell as described elsewhere [ 3 ]. A water-vapor-saturated argon gas flow was maintained over the cell to prevent oxidation and maintain a near constant pH20 at 320"C. The equilibrium between the molten flux and the partial pressure of water in the gas had been reported by Rahmel [ 13 ]. Under these acidic conditions, dark brown crystals were formed on the anode in 3 h of electrolysis when the current density was controlled at 1 mA/cm 2. An SEM micrograph of representative crystallites is shown in Fig. 2(a). The CuO structure of these crystallites was confirmed by X-ray diffraction analysis as shown in Fig. 3 (a). For the basic conditions experiment, water is removed by maintaining the molten flux at 450"C for 12 h in a quartz cell under a stream of dry argon. After complete dehydration, the platinum crucible with the charge of oxides materials is quickly transferred into the vacuum chamber. Electrolysis at 450"C of the basic melt with 1 mA/cm 2 current density gave a rapid deposition of plate-like crystals shown in Fig. 2(b) which proved to be La2_xNaxCuO4. A current density greater than 5 mA/cm 2 results in CuO crystallite deposition on the anode, indicating that the diffusion of La complex ions is important. After 2 h of electrolysis at 1 mA/cm 2 constant current, the average crystal size is approximately 100 ~tm as shown in Fig. 2(c). X-ray diffraction studies shown in Fig. 3(b) indicate that the crystallites contain a La2CuO4-related phase with the growth direction along the a-bplane. Averaging the EDX analysis on several crystals, yields a composition in atom percent of the crystals of 66.8% La and 33.2% Cu which is in good agreement with ICP-AES results. At the low concentration of alkali metals in this system, we do not consider the EDX results reliable for sodium determination. Same as in the ICP-AES analysis, the accuracy of elemental sodium may not be well represented by the precision of the measurement. However, an x value smaller than 0.08 is consistently obtained in all of the ICP-AES analysis. Fig. 4 displays the typical SQUID data of the asgrown crystals which are harvested by scratching crystals out from the electrode surface. The results show that the samples are superconducting with an onset temperature ~ 32 K. The estimated superconducting volume fraction is more than 60%, suggest-
327
(a)
(b)
(c) Fig. 2. Photomicrographsof (a) CuO (b) La=_=Na=CuO4+=prepared by 10 rain of electrolysis, (c) La2_xNaxCuO4+xprepared by 2 h of electrolysisdepositedon the electrodesurface. ing bulk superconductivity of the crystals. However, sample compositional inhomogeneity exists as exemplified by the rather broad transition and the paramagnetic-like contribution to the susceptibility in the field-cooled measurement. Cyclic voltammetry is not
H.Y. Tang et al. / Physica C231 (1994) 325-329
328
L a 2 _ x N a x f u O 4 in the deposit, indicating that diffu-
2500
sion of lanthanum complex ions acts as a fundamental step in the superconducting phase formation. Higher molecular weight hydroxide flux deposition does not appear practical in view of our studies of the electrochemical crystal growth of this material. A similar study performed using a KOH molten flux has not been found to have satisfactory results for La2_xKxCuO4 deposition. The intrinsic difference of electrochemical oxidation ability between KOH and NaOH may significantly impact the deposition mechanism.
2250 2000 1750 CuO
<-~ 1500 .~ 1250
2
lOOO
750 500 250 0 10
20
30
40
50
60
70
80
90
4. Conclusion
2 theta Fig. 3. Typical powder X-ray diffraction patterns of (a) CuO (b) La2_xNaxCuO4.x crystallites.
0890000 ~.
-2
o
70
o
oF C °
o
o o o o
-4
Crystallization of La2_ xNaxCuO4by a low-temperature, isothermal electrochemical method has been demonstrated. This technique offers the distinct advantage of superconductor synthesis through electrochemical control and has considerable potential for the synthesis of ternary or even multinary high-To materials. In this preliminary research, temperature, pH20, flux composition, and electrolytic conditions are correlated with product formation. Detailed studies to clarify electrochemical interaction among these factors are proceeding.
0
-6 0
ZFC
Acknowledgements
0
H=20G
-8
0 0 -I0
i
I0
i 20
i
30
40
T e m p e r a t u r e (K)
The authors thank S.M. Rao and M.L. Norton for helpful discussion. This work is supported by the ROC National Science Council Grant NSC83-0511M-007-004.
Fig. 4. SQUID measurement for the La2_xNaxCuO4+x crystals grown from low-temperature molten flux.
able to provide valuable information about the possible electrochemical reaction mechanism in both the acidic and basic experiments due to reference electrode instability and low decomposition potential in the basic environment. Potentiostatic deposition experiments were performed at 0.075 V in the basic melt with a La203 to CuO molar ratio range from 2 to 10. La2_xNa~CuO4 and CuO mixed-phase growth at the electrode surface was found in all cases after 2 h of electrolysis. Higher La/Cu molar ratios display more
References [1] R.N. Bhattacharya, R. Noufi, L.L. Roybal and R.K. Ahrenkiel, J. Electrochem. Soe. 138 ( 1991 ) 1643. [2] J.M. Rosamilia and B. Miller, J. Electrochem. Soc. 136 (1989) 1053. [3] M.L. Norton and H.Y. Tang, Chem. Mater. 3 ( 1991 ) 431. [4] P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phy. Rev. Lett. 58 (1987 ) 2482. [ 5 ] J.C. Bennett, M. Olfert, G.A. Seholz and F.W. Boswell, Phys. Rev. B 44 (1991) 2727.
H.Y. Tang et aL /Physica C 231 (1994) 325-329 [6]J.C. Grenier, A. Wattiaux, N. Lagueyte, J.C. Park, E. Marquestaut, J. Etoumeau and M. Pouchard, Physica C 173 (1991) 139. [7] J.G. Bednorz, M. Takashige and K.A. Muller, Europhys. Lett. 3 (1987) 379. [8] G. Demazeau, F. Tresse, Th. Plante, B. Chevalier, J. Etourneau, C. Michel, M. Hervieu, B. Raveau, P. Lejay, A. Sulpice and R. Tournier, Physica C 153 (1988) 524.
329
[9] M.A. Subramanian, J. Gopalakrishnan, C.C. Torardi, T.R. Askew, R.B. Flippen, A.W. Sleight, J.J. Lin and S.J. Pooh, Science 240 (1988) 495, [10] W.K. Ham, G.F. Holland and A.M. Stacy, J. Am. Chem. Soc. 110 (1988) 5214. [ 11 ] J. Goret, Bull. Soc. Chim. (1974) 1074. [ 12] J. Goret and B. Tremillon, Bull. Soc. Chim. (1966) 67. [ 13 ] A. Rahmel and H.J. Kruger, Z. Phys. Chem. 55 (1967) 25.